We are currently studying two biological structures: crystalline actin sheets and the Sendai virus nucleocapsid. The purpose of the first project is to determine the molecular structure of actin to at least 10 A resolution. The crucial prerequisite for the realization of this project is the existence of highly ordered thin crystalline sheets of actin prepared by dialysis of G-actin against gadolinium ions. The specific aims are: To obtain 3-dimensional model of actin to 15 A resolution, determined from electron micrographs from tilted views of negatively stained preparations of crystalline actin sheets, and to find the orientation of the actin molecule within the actin filament by computationally aligning the model obtained from actin sheets with the model derived from paracrystals of actin filaments. The nucleocapsids of enveloped viruses are structures of major importance both as model-systems for the study of the synthesis and transport of proteins within cells, and in their own right as vital components of pathogenic viruses. The purpose of our project is to study the structure of the Sendai virus nucleocapsid. We aim to determine the geometrical packing of protein within the nucleocapsid and the location of the single RNA strand.